1. Field of the Invention
The present invention concerns a digital/analog converter which is more particularly designed to work with microwaves because of its simple structure which enables digital/analog conversion at very high throughput rates. The digital/analog converter according to the invention has an output stage which stabilizes the analog voltage delivered. Its embodiment is more particularly useful in fast integrated circuit form made by means of heterojunction transistors of Group III-V materials, such as GaAs and Al GaAs for example.
2. Description of the Prior Art
There are known methods for converting a binary number into an analog value by means of several programmed, parallel-mounted current sources Each source is controlled by one of the bits of the binary number. A current/voltage converting transistor adds up the currents of all the sources and converts the resultant current into an analog voltage.
The greater the number of bits of the input binary signal, the more precise is this voltage. However, it is subject to phenomena of instability, the sources of which can be found in all the components of the converter. This instability includes the instability of two voltage sources, +VDD and -VSS, between which the converter is supplied with power, the instability of the transistors of the programmed current sources and the instability of the current/voltage converter.
An object of the invention, therefore, is to propose a digital/analog converter, the output voltage of which is stabilized by a circuit which has a diagram resembling a BFL gate but which works differently, the output of this circuit being looped to the gate of the inverting transistor of the first stage, which forms the current/voltage converter. The analog voltage collected at the drain of this inverting transistor is addressed to the gate of a transistor mounted as a follower source in a second shifter gate which further has a shifter diode and a pull-back transistor. The output voltage of the digital/analog converter, taken at the common point between the shifter diodes and the pull-back transistor, is looped to the gate of the inverting transistor which is the current/voltage converter. Since it is an inverter, if the output voltage develops in one direction, the inverting transistor makes it develop in the reverse direction and stabilizes it.